"Burn-in" is a reliability screening method used to anticipate the failure of semiconductors, e.g., integrated circuit chips, by exposing them, prior to their actual use in products, to elevated temperatures usually on the order of 125.degree.-150.degree. C. By exposing integrated circuit chips to such elevated temperatures, their normal "aging process" is accelerated in order to cause failure of those chips which are inherently defective. That is to say that most chips that fail will do so within the first few months of use; hence, burn-in screenings are employed to accelerate the stress conditions which will be experienced by a chip during that prospective period. Any chip that survives this burn-in screening is expected to be 99.9% failure free until it dies of "old age", earned by actual use in a product.
In the past, two different methods have been employed to burn-in screen integrated circuit chips. The first method is usually referred to as "oven burn-in." It employs an oven-like device and an air or nitrogen atmosphere wherein chips are exposed to heat and voltage stress over an extended period of time. Overheating has, however, proven to be a major problem with this method since air has poor heat transfer capabilities. That is to say that during such oven burn-in procedures, the heat needed to properly screen the integrated circuit chip cannot be dissipated to the surrounding atmosphere quickly enough to prevent damage to the chips once the chip has been heated sufficiently to conduct the burn-in screening procedure. Heat sinks are therefore employed in the operation of such burn-in ovens. However, temperature uniformity from device to device is poor. Moreover, even when heat sinks are so employed, the poor temperature stability and/or oxidation of components in the oven's atmosphere will often cause other problems. Oven burn-in reliability screening is also not practical for high-power, TAB-bonded integrated circuit chips due to their high power densities.
The other prior art burn-in reliability screening method is often referred to as "liquid immersion burn-in." It usually involves the complete immersion of the chip and a chip connector or socket --a device with which the chip needs to be associated to perform the burn-in screening procedure--into a liquid immersion bath heated to a desired temperature. Two inherent problems are generally associated with liquid immersion burn-in screenings. The first problem is that the immersion fluids--if contaminated--are likely to contaminate successive chips undergoing screening. Consequently, complex and, hence, expensive filtering systems are required for the repeated use of such baths. Furthermore, those chip connectors or sockets normally associated with such chips during liquid burn-in screenings tend to trap some of the immersion fluids in their many cavities. This causes a considerable loss of the expensive fluids or a considerable waste of time waiting for them to "drip off" once they are removed from the bath. The immersion fluids used to make up these baths are also extremely expensive. However, the liquid burn-in method does have an advantage over the oven burn-in method in that the liquid bath can also serve as a heat sink which is capable of quickly drawing heat energy away from the chip and its connector at the appropriate point in the burn-in procedure to thereby prevent any undesired overheating of the chip.
Yet another problem with both of these burn-in methods follows from the fact that the chip connector or socket, which is used to physically hold and make electrical contact with a chip during burn-in screening, and the printed circuit board that the socket is attached to, are exposed to the very same heat conditions needed to screen the integrated circuit chip. This exposure requires that the connector and circuit board be made of an expensive material and provided with special soldering for electrical connections which are capable of surviving the relatively high temperatures (e.g., 125.degree.-150.degree. C.) usually required to burn-in screen an integrated circuit chip. That is to say that an entire chip and connector, i.e., the connector or socket plus the chip, or the printed circuit board, will fail a burn-in in screening procedure--as opposed to failure of the chip alone--if the connector or printed circuit board themselves fail for any reason. Consequently, failure of the connector or printed circuit board requires that the reliability screening procedure be repeated for the chip associated with the connector or printed circuit board at the time of its failure. This can only be done at the considerable additional labor expense associated with removing the chip from the failed connector or socket and remounting the chip in a new connector for a repeated screening procedure.
The primary advantage of applicant's burn-in method is that it makes the burn-in screening of relatively high power, TAB-bonded integrated circuit chips feasible. This feasibility is important because, owing to the high power density in a bare chip, TAB-bonded package, oven burn-in is not practical. Immersion burn-in may work but it has the many disadvantages previously noted. By employing applicant's reliability screening procedure, PC boards do not have to be removed from the system (as in oven burn-in) and burn-in screening can be carried out for socketed integrated circuit chips in a low temperature ambient environment.
The herein described methods and apparatus avoid each of the above mentioned problems by providing a method for burn-in screening an integrated circuit chip without directly exposing its connector or the printed circuit board to the heat conditions they would otherwise encounter in a burn-in oven or in an immersion bath used to conduct liquid burn-in screening procedures. That is to say that, unlike either the oven burn-in or the liquid burn-in screening procedure, the connector and printed circuit board used in applicant's process are, to a large extent, isolated from direct contact with the source of the high temperatures needed to burn-in screen the chip. Thus, applicant's connector or socket and printed circuit boards need not be made of expensive heat-resistant materials and soldered electrical connections need not be constructed of materials capable of withstanding repeated exposure to 125.degree.-150.degree. C. temperatures. Consequently, a chip connector's useful life--regardless of the material used to make it and the life of the printed circuit board, are extended because they are somewhat removed from direct physical contact with the source of heat employed to screen the chip. Also, because applicant's method employs a closed loop heat transfer fluid system whose fluid never touches the chip itself, there is little or no fluid loss and/or fluid contamination. Likewise, better (e.g., more instantaneous) overall temperature control of the entire screening procedure can be achieved with applicant's burn-in screening procedure and/or apparatus.